The purpose of this study is to analyze and assess the zone comprised of the top of the storage formation rock and bottom of the overlying confining zones (known as the confining zone/formation interface, Figure 1) surrounding CO2 injection wells. Effective confining zones are typically low-permeability rock units that are relatively thick and serve as a seal above the storage formation rock that prevents any injected CO2 from migrating upwards out of the storage formation. The study is using information obtained from rock outcropings and core samples of proposed CO2 clastic formations and overlying mudstone confining zones to determine the characteristics of this interface zone, and is using these data to develop conceptual models of the behavior of formation-confining zone interfaces and, in particular, sandstone/mudstone contacts.
The New Mexico Institute of Mining and Technology (NMIMT), in collaboration with Utah State University and Sandia National Laboratories, is examining the primary depositional texture of the sediment at the interface, the structural characteristics of the rock within this zone, and the changes that the sediment underwent as it transformed into rock and following rock formation (diagenesis). These elements affect how the rock responds physically and chemically to CO2 injection. Of concern is the possibility that mesoscale (centimeters to several meters) features at the interface—such as zones where the rock interfaces are preferentially cemented (diagenetic), or is deformed (e.g., fractures)—may greatly influence the degree that the formation and confining zone are connected (Figure 2). High connectivity may in some cases provide leakage pathways into the confining zone, compromising its ability to provide a long-term seal for stored CO2.
The overall goal of the Department of Energy’s (DOE) Carbon Storage Program is to develop and advance technologies that will significantly improve the effectiveness of geologic carbon storage, reduce the cost of implementation, and prepare for widespread commercial deployment between 2020 and 2030. Research conducted to develop these technologies will ensure safe and permanent storage of carbon dioxide (CO2) to reduce greenhouse gas (GHG) emissions without adversely affecting energy use or hindering economic growth.
Geologic carbon storage involves the injection of CO2 into underground formations that have the ability to securely contain the CO2 permanently. Technologies being developed for geologic carbon storage are focused on five storage types: oil and gas reservoirs, saline formations, unmineable coal seams, basalts, and organic-rich shales. Technologies being developed will work towards meeting carbon storage programmatic goals of (1) estimating CO2 storage capacity +/- 30 percent in geologic formations; (2) ensuring 99 percent storage permanence; (3) improving efficiency of storage operations; and (4) developing Best Practices Manuals. These technologies will lead to future CO2 management for coal-based electric power generating facilities and other industrial CO2 emitters by enabling the storage and utilization of CO2 in all storage types.
The DOE Carbon Storage Program encompasses five Technology Areas: (1) Geologic Storage and Simulation and Risk Assessment (GSRA), (2) Monitoring, Verification, Accounting (MVA) and Assessment, (3) CO2 Use and Re-Use, (4) Regional Carbon Sequestration Partnerships (RCSP), and (5) Focus Area for Sequestration Science. The first three Technology Areas comprise the Core Research and Development (R&D) that includes studies ranging from applied laboratory to pilot-scale research focused on developing new technologies and systems for GHG mitigation through carbon storage. This project is part of the Core R&D GSRA Technology Area and works to develop technologies and simulation tools to ensure secure geologic storage of CO2. It is critical that these technologies are available to aid in characterizing geologic formations before CO2-injection takes place in order to predict the CO2 storage resource and develop CO2 injection techniques that achieve optimal use of the pore space in the reservoir and avoid fracturing the confining zone (caprock). The program’s R&D strategy includes adapting and applying existing technologies that can be utilized in the next five years, while concurrently developing innovative and advanced technologies that will be deployed in the decade beyond. This research project is obtaining a better knowledge base of the characteristics of the interface between the confining zones above the storage formation and the storage formation rock itself. Increased understanding in this area will allow for better selection of storage sites that can effectively contain CO2 in the formation.
This work benefits the carbon capture and storage community by advancing the understanding of dynamics specific to the confining zone/formation interface and improving models used by the carbon capture and storage community to predict and characterize potential storage formations. This contributes to the Carbon Storage Programmatic goal of demonstrating 99 percent storage permanence by better understanding the conditions encountered at the interface of the confining layer and storage formation. Additionally, training of future carbon capture and storage professionals is enhanced through opportunities for interaction between personnel at Sandia National Laboratories and students and faculty at Utah State University and NMIMT.
This study is assessing depositional, structural, and diagenetic characteristics of confining zone/formation interfaces of proposed CO2 injection formations, and how insight gained from observation linked to coupled modeling can guide inferences into best modeling practices. NMIMT is examining outcrop analogs and core samples of proposed CO2 clastic formations and overlying mudstone confining zone and performing laboratory testing to properly characterize the thermal, hydrological, chemical, and mechanical properties of sandstone-mudrock interfaces. Some of the specific topics addressed in this study include:
How physical properties of sand/mudstone interfaces influence CO2 storage and transport.
How geochemical perturbations.
Induced by CO2 emplacement influence leakage of CO2 across the interface.
How the physical/chemical properties at the interface affect brine migration into confining zones (which could mitigate pressure issues that result from injecting CO2 into the brine saturated formation).
How fractures at the interface respond to increased pressures resulting from CO2 injection into the formation rock.
NMIMT is investigating interfaces in three different formation/seal pairs. The first stage of the project involved a brief field campaign to perform basic description of the interfaces and conduct detailed sampling. The second stage involves laboratory analysis of the samples to identify their key petrographic, geochemical, mechanical, and petrophysical properties. Finally, the descriptive information acquired in the first two steps is being used as input data for a detailed coupled thermal, hydrologic, mechanical, and chemical modeling investigation of the implication for transmission of fluids across the interface. The modeling effort includes generating a range of numerical experiments that tests how stresses and fractures are transmitted across the sedimentary interface. NMIMT is examining a range of conditions for fracture propagation across a range of boundaries, including narrow bonded, narrow contact with free slip, and an interface with a finite width.
The success of this project depends heavily on successfully classifying field work observations into usable objective functions in terms of how observed sandstone-mudrock interfaces are expected to behave during CO2 injection and storage, and the ability to integrate this understanding in terms of coupled thermal, hydrological, mechanical, and chemical (THMC) models.
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